Rapid convergence of time-averaged frequency in phase synchronized systems

Numerical and experimental evidence is presented to show that many phase synchronized systems of non-identical chaotic oscillators, where the chaotic state is reached through a period-doubling cascade, show rapid convergence of the time-averaged frequency. The speed of convergence toward the natural frequency scales as the inverse of the measurement period. The results also suggest an explanation for why such chaotic oscillators can be phase synchronized.Comment: 6 pages, 9 figure

Simple model for 1/f noise

We present a simple stochastic mechanism which generates pulse trains exhibiting a power law distribution of the pulse intervals and a $1/f^\alpha$ power spectrum over several decades at low frequencies with $\alpha$ close to one. The essential ingredient of our model is a fluctuating threshold which performs a Brownian motion. Whenever an increasing potential $V(t)$ hits the threshold, $V(t)$ is reset to the origin and a pulse is emitted. We show that if $V(t)$ increases linearly in time, the pulse intervals can be approximated by a random walk with multiplicative noise. Our model agrees with recent experiments in neurobiology and explains the high interpulse interval variability and the occurrence of $1/f^\alpha$ noise observed in cortical neurons and earthquake data.Comment: 4 pages, 4 figure

Gegen den Brain Drain des wissenschaftlichen Nachwuchses: Reformvorschläge zur Hochschulpolitik in Deutschland

Seit langem beklagt der wissenschaftliche Nachwuchs, dass das deutsche Hochschulsystem kaum langfristige Karriereperspektiven und wenig individuelle Freiheiten bietet. Auf viele Jungwissenschaftler wirkt die Realität des deutschen Hochschulbetriebs abschreckend. Was müsste sich ändern? Vorschläge der German Scholars Organization (GSO)

HUT observations of carbon monoxide in the coma of Comet Levy (1990c)

Observations of comet Levy (1990c) were made with the Hopkins Ultraviolet Telescope during the Astro-1 Space Shuttle mission on 10 Dec. 1990. The spectrum, covering the wavelength range 415 to 1850 A at a spectral emission of 3 A (in first order), shows the presence of carbon monoxide and atomic hydrogen, carbon, and sulfur in the coma. Aside from H I Lyman-beta, no cometary features are detected below 1200 A, although cometary O I and O II would be masked by the same emissions present in the day airglow spectrum. The 9.4 x 116 arcsec aperture corresponds to 12,000 x 148,000 km at the comet. The derived production rate of CO relative to water, 0.13 + or - 0.02, compared with the same ratio derived from IUE observations (made in Sep. 1990) which sample a much smaller region of the coma, 0.04 + or - 0.01, suggests the presence of an extended source of CO, as was found in comet Halley. Upper limits on Ne and Ar abundance are within an order of magnitude or solar abundances

The Very Highly Ionized Broad Absorption Line System of the QSO SBS1542+541

We have analyzed the broad absorption line system of the bright (V=16.5) high-redshift (z=2.361) QSO SBS1542+541 using UV spectra from the HST FOS along with optical data from the MMT and the Steward Observatory 2.3m telescope. These spectra give continuous wavelength coverage from 1200 to 8000 Angstroms, corresponding to 340 to 2480 Angstroms in the QSO rest frame. This object therefore offers a rare opportunity to study broad absorption lines in the rest-frame extreme UV. We find that the absorption system is dominated by very high-ionization species, including O VI, NeVIII, and SiXII. We also identify apparently saturated broad Lyman-series lines of order Ly-gamma and higher. There is strong evidence for partial occultation of the QSO emission source, particularly from the higher-order Lyman lines which indicate a covered fraction less than 0.2. Overall, the data suggest a correlation between a larger covered fraction and a higher state of ionization. We suggest that the different covered fractions can be explained by either a special line of sight through a disk-like geometry or by the existence of density fluctuations of a factor >2 in the BAL gas. Our photoionization models of the system indicate a large column density and high ionization state similar to that found in X-ray ``warm absorbers''.Comment: 31 pages, 13 figures, to be published in Ap

A Composite HST Spectrum of Quasars

We construct a composite quasar spectrum from 284 HST FOS spectra of 101 quasars with redshifts z > 0.33. The spectrum covers the wavelengths between 350 and 3000 A in the rest frame. There is a significant steepening of the continuum slope around 1050 A. The continuum between 1050 and 2200 A can be modeled as a power law with alpha = -0.99. For the full sample the power-law index in the extreme ultraviolet (EUV) between 350 and 1050 A is alpha = -1.96. The continuum flux in the wavelengths near the Lyman limit shows a depression of about 10 percent. The break in the power-law index and the slight depression of the continuum near the Lyman limit are features expected in Comptonized accretion-disk spectra.Comment: 10 figures To appear in the February 1, 1997, issue of the Ap.

ASCA Observations of the Composite Warm Absorber in NGC 3516

We obtained X-ray spectra of the Seyfert 1 galaxy NGC~3516 in March 1995 using ASCA. Simultaneous far-UV observations were obtained with HUT on the Astro-2 shuttle mission. The ASCA spectrum shows a lightly absorbed power law of energy index 0.78. The low energy absorbing column is significantly less than previously seen. Prominent O~vii and O~viii absorption edges are visible, but, consistent with the much lower total absorbing column, no Fe K absorption edge is detectable. A weak, narrow Fe~K$\alpha$ emission line from cold material is present as well as a broad Fe~K$\alpha$ line. These features are similar to those reported in other Seyfert 1 galaxies. A single warm absorber model provides only an imperfect description of the low energy absorption. In addition to a highly ionized absorber with ionization parameter $U = 1.66$ and a total column density of $1.4 \times 10^{22}~\rm cm^{-2}$, adding a lower ionization absorber with $U = 0.32$ and a total column of $6.9 \times 10^{21}~\rm cm^{-2}$ significantly improves the fit. The contribution of resonant line scattering to our warm absorber models limits the Doppler parameter to $< 160~\rm km~s^{-1}$ at 90\% confidence. Turbulence at the sound speed of the photoionized gas provides the best fit. None of the warm absorber models fit to the X-ray spectrum can match the observed equivalent widths of all the UV absorption lines. Accounting for the X-ray and UV absorption simultaneously requires an absorbing region with a broad range of ionization parameters and column densities.Comment: 14 pages, 4 Postscript figures, uses aaspp4.sty To appear in the August 20, 1996, issue of The Astrophysical Journa

A Far-Ultraviolet View of Starburst Galaxies

Recent observational and theoretical results on starburst galaxies related to the wavelength regime below 1200 A are discussed. The review covers stars, dust, as well as hot and cold gas. This wavelength region follows trends similar to those seen at longer wavelengths, with several notable exceptions. Even the youngest stellar populations show a turn-over in their spectral energy distributions, and line-blanketing is much more pronounced. Furthermore, the O VI line allows one to probe gas at higher temperatures than possible with lines at longer wavelengths. Molecular hydrogen lines (if detected) provide a glimpse of the cold phase. I cover the crucial wavelength regime below 912 A and the implications of recent attempts to detect the escaping ionizing radiation.Comment: 8 pages, 3 figures, Invited Talk, Starbursts--From 30 Doradus to Lyman-Break Galaxies, ed. R. de Grijs & R. M. Gonzalez Delgado (Dordrecht: Kluwer